Antioxidant control of leachable mercury in fluorescent lamps

ABSTRACT

The formation of leachable mercury upon disposal or during TCLP testing of mercury vapor discharge lamps is substantially prevented by incorporation of an antioxidant in the lamp structure or in the test solution.

This invention is directed to mercury vapor arc discharge lamps in whichthe arc discharge takes place in mercury vapor, including conventionalphosphor fluorescent lamps and more particularly to avoidance of mercurypollution of landfills and groundwaters upon disposal of such lamps andduring testing for leaching of toxic materials from such lamps and tocompositions of matter useful in preventing the formation of leachablemercury in disposal and testing procedures. The lamps provided hereinare characterized by reduced solubilization and leaching of mercury whenthe lamp is pulverized for testing or upon disposal.

BACKGROUND OF THE INVENTION

Low pressure mercury arc discharge lamps are standard lighting meanswhich include electrodes sealed in a glass envelope, the interior ofwhich may be coated with a phosphor. The lamp also contains a smallamount of mercury and an inert gas at low pressure, of about 1 to 5torr. The term lamp, as used herein, means the complete unit includingthe glass envelope and the end pieces and plugs for mounting in a lampfixture, and wires which connect the internal components of the envelopewith the end pieces.

During manufacture of fluorescent or low pressure mercury arc lamps anamount of elemental mercury (Hg^(O)) is sealed in the lamp envelope.Most of the mercury adheres to the phosphor coating, a small amountbeing in the vapor phase.

During operation, alkali metal carbonates from the electrodes decomposeand form free oxygen in the lamp. The oxygen may react with a portion ofthe mercury to form soluble mercury oxide (Hg^(O)). Soluble mercuryoxide is leachable from land fills and other disposal facilities.Soluble mercury oxides or other oxidized forms of mercury formed in thecourse of the test are detrimental to the accuracy and reliability ofthe standard test for determination of the leachability of toxicmaterials from lamp waste. This test is generally referred to as theToxicity Leaching Characteristic Procedure or TLCP test.

There is concern about the environmental impact of soluble mercurycompounds which can leach into ground water sources, rivers, streams,and the like.

SUMMARY OF THE INVENTION

Ferric and cuprous ions form soluble compounds which are capable ofoxidizing elemental mercury to the monovalent, mercurous, form which issoluble in an acidic aqueous environment and therefore leachable. Theformation of ferric and cuprous compounds depend on exposure to andreaction with oxygen. Antioxidants, either organic or inorganic,incorporated in the lamp prevent formation of ferric and cuprouscompounds, in the presence of water or moisture, by oxidation of ironand copper from lamp components, thereby greatly reducing or preventingthe formation of leachable mercurous and mercuric compounds by oxidationof elemental mercury.

The invention provides a mercury vapor discharge lamp comprising anenvelope of light transmitting glass which contains, an inert gas and anamount of elemental mercury, a pair of electrodes for establishing anarc discharge, and an effective amount of an antioxidant.

The lamp further comprises at least one base or end cap which defines acavity having an inner surface, and which is secured to the lampenvelope by a basing cement, the antioxidant being disposed within saidcavity. Generally such lamps have a pair of end caps.

The antioxidant is admixed with the basing cement securing the end capsin place on the glass envelope.

The invention further provides a composition which can be included inthe lamp structure for the purpose of controlling oxidation of iron andcopper in order to minimize or prevent formation of soluble mercurycompounds, thereby reducing or preventing formation of water solubleleachable mercury compounds in landfills or TLCP test samples.

In a preferred embodiment of the invention, the mercury vapor dischargelamp is provided with an antioxidant carried on the inner surface of thecavity by means of an inert water soluble adhesive binder.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially broken away section of a low pressure mercuryfluorescent lamp 1 comprising a glass envelope 2 having a phosphorcoating 9 on the inner surface of the glass. Connector pins 6 are inelectrical contact with leads 4 and 5 and also with electrodes 3.

In FIGS. 2 and 3 the end caps are shown as 2, corresponding to numeral 8of FIG. 1. The end cap is provided with connector pins 6 and pH controlagent 8 deposited in the cavity of end cap 2.

DESCRIPTION OF THE INVENTION

The incorporation of antioxidants in a lamp structure or to the TLCPtest solution prevents oxidation of iron and copper metal components toa form which is both soluble and capable of oxidizing elemental mercuryto a soluble form of mercury oxide. Accordingly the formation anddissolution of soluble ferric and cuprous compounds from the lampcomponents is diminished or prevented resulting in reduction orprevention of leachable mercury compounds.

The formation of leachable mercury when fluorescent lamps are broken andexposed to landfill conditions can be prevented or minimized bypreventing oxidation of certain components of the lamp. Certain metalcomponents of fluorescent lamps particularly iron lead wires, coppercoated leads, and any brass components generate ferric (Fe⁺³) andcuprous (Cu⁺¹) ions when exposed to moisture, oxygen, and acidity.

In order to address the growing concern that excessive amounts ofmercury from disposal of fluorescent lamps might leach into surface andsubsurface bodies of water, the Environmental Protection Agency hasestablished a maximum concentration level for mercury at 0.2 milligramsof leachable mercury per liter. This is generally determined by thestandard analysis known as the Toxicity Characteristic LeachingProcedure (TCLP), a well known test procedure.

In carrying of the TCLP test, the lamps are pulverized to form lampwaste material similar to that which would result from lamp disposal inland fills or other disposal locations. The ambient conditions in suchlocations may be such as to promote formation of leachable mercury justas the TCLP test conditions themselves tend to allow for formation ofleachable mercury in amounts greater than the established limit of 0.2milligrams per liter.

It has been found that elemental mercury added to mercury-freepulverized lamp materials prepared for the TCLP test is converted toleachable mercury in the course of the test. If elemental mercury aloneor in combination with various glass, phosphor, or non-metal lampcomponents is tested, little or essentially no leachable mercury isfound. When elemental mercury is tested in combination with metal lampcomponents such as copper or iron, lead wires, pins, or other metalhardware, the mercury is transformed into a leachable form.

It was determined by controlled experimentation that both ferric iron(trivalent) and cuprous (monovalent) copper are generated under the TCLPtest conditions when carried out in the presence of oxygen and thatthese ionic species are able to oxidized elemental mercury to solublemercury compounds which are measured as leachable mercury.

Corrosion or dissolution of metals from the metallic state requires thepresence of both oxygen and a solvent such as water conditions thatexist in the TCLP test and landfill situations. Accordingly, it has beenfound that the formation can be controlled or prevented by controllingor excluding exposure to oxygen of the iron and copper-containing metallamp components. This can be done by the use of oxygen-free or anaerobictest and disposal conditions.

Organic or inorganic antioxidants incorporated into fluorescent lampsduring manufacture become operative in the course of preparing lamps forthe TCLP test or upon destruction of the lamp during disposal. Thepresence of such antioxidants will make the TCLP test more accurate andreliable and will reduce the formation of soluble mercury compounds whenthe lamps are disposed of.

Suitable antioxidants include any materials, compounds, or systems whichprevent or reduce the formation of ferric and cuprous ions in themercury-containing environment. Illustrative organic and inorganicantioxidants (reducing agents) include ascorbic acid, sodium ascorbate,calcium ascorbate, ferrous sulfate, ferrous oxide, ferrous tatrate,ferrous citrate, ferrous gluconate, ferrous chloride , and the like.Ascorbic acid is a preferred antioxidant for use in this invention.

The principles and practice of this invention will be more fullyunderstood when considered in view of the following examples.

All TCLP test data was obtained by the test procedure prescribed onpages 26987-26998 volume 55, number 126 of the Jun. 29, 1990 issue ofthe Federal Register.

Briefly, lamps being tested are pulverized into particulate form havingthe prescribed particle size which is capable of passing through 3/8inch sieve. The test material is then extracted with a sodiumacetate-acetic acid buffer at a pH of about 4.93.

To prevent the spurious formation of leachable mercury upon disposal ofmercury vapor discharge lamps and to improve the reliability of the TCLPtest an effective amount of an antioxidant is incorporated in the lampstructure, for example within the glass envelope exterior to the plasmadischarge or in an end-cap, or in the base of the lamp. An effectiveamount of the antioxidant is that amount which will substantiallyprevent formation of ferric and cuprous compounds which can oxidizeelemental mercury to a soluble form. In general, an effective amount ofthe antioxidant will be enough for the TCLP test results to show thepresence of less than about 0.2 parts per million of leachable mercury.

The formation of soluble mercury compounds is illustrated by the data inTable 1, below. Carrying out the TCLP test in the presence of airgenerates about 1 part per million of copper and about 0.3 parts permillion of soluble iron. The amount of soluble mercury formed underthese conditions exceeds the regulatory limit of 0.2 parts per million.Increasing the exposure to oxygen increases the amount of soluble copperand soluble mercury formed. Decreasing exposure to oxygen decreases theformation of soluble copper, soluble iron, and soluble mercury.

                  TABLE 1                                                         ______________________________________                                        Gas Type    Soluble Cu ppm                                                                            Soluble Hg ppm                                        ______________________________________                                        Air         1.07        0.777                                                 Argon       0.06        <0.050                                                Oxygen      3.04        1.030                                                 ______________________________________                                    

When the amount of oxygen is varied by increasing the volume of the headspace in the TCLP test jar, the effect of both soluble iron and copperon the formation of soluble mercury is evident from the data in Table 2,below. As the head space volume increases, the amount of soluble mercuryincreases in response to the formation of increasing amounts of solublecopper and iron.

                  TABLE 2                                                         ______________________________________                                                     Soluble              Soluble                                     Head Space   Mercury    Soluble Iron                                                                            Copper                                      (mL)         (ppb)      (ppm)     (ppm)                                       ______________________________________                                        0      0.0000    210        3.62    0.35                                      1      140       214        4.63    0.40                                      2      205       203        5.04    0.63                                      3      360       250        5.22    0.43                                      4      494       311        5.22    0.51                                      5      763       525        6.13    1.04                                      6      1013      458        5.80    1.02                                      7      1508      583        8.12    1.13                                      ______________________________________                                    

When an antioxidant such as ascorbic acid is added to the test solution,the formation of soluble mercury is decreased as shown in Table 3,below.

                  TABLE 3                                                         ______________________________________                                                        Amount of                                                            Hg Dose  Additive         Final  Leachable                             Additive                                                                             (mg/lamp)                                                                              (gms/lamp)                                                                              ORP*   pH     Hg (ppb)                              ______________________________________                                        none   21.2     --        --     --     745                                   ascorbic                                                                             20.0     2         130    5.2    <50                                   acid                                                                          ascorbic                                                                             20.0     0.1                     134                                   acid                                                                          ascorbic                                                                             20.0     0.2                     72                                    acid                                                                          ascorbic                                                                             20.0     0.5                     52                                    acid                                                                          ascorbic                                                                             20.0     1                       84                                    acid                                                                          sodium 20.0     1                       32                                    ascorbate                                                                     sodium 20.0     0.2                     51                                    ascorbate                                                                     sodium 20.0     0.5                     46                                    ascorbate                                                                     sodium 20.0     2                       43                                    ascorbate                                                                     calcium                                                                              20.0     0.2                     56                                    ascorbate                                                                     calcium                                                                              20.0     0.5                     95                                    ascorbate                                                                     calcium                                                                              20.0     1                       76                                    ascorbate                                                                     calcium                                                                              20.0     2                       59                                    ascorbate                                                                     none   20.0     0                       586                                   Ferrous                                                                              20.0     1                       149                                   sulfate                                                                       Ferrous                                                                              20.0     2                       112                                   sulfate                                                                       Ferrous                                                                              20.0     0.5                     318                                   sulfate                                                                       Ferrous                                                                              20.0     0.2                     443                                   sulfate                                                                       Ferrous                                                                              20.0     1                       97                                    gluconate                                                                     Ferrous                                                                              20.0     2                       75                                    gluconate                                                                     Ferrous                                                                              20.0     0.4                     281                                   gluconate                                                                     ______________________________________                                         *ORP = Oxidation Reduction Potential                                     

Table 3 shows examples of antioxidants that have been added to the TCLPextraction to study the effect upon leachable mercury. The first entryis a control showing the amount of leachable mercury generated when thepulverized lamp is dosed with 20 mg of elemental mercury. The leachablemercury generated in that case is 745 ppb in the absence of anyantioxidant. The antioxidant can be incorporated in the lamp byencapsulation of the material in a glass capsule that can be placedeither in the base of the lamp between the aluminum cap and flare ofleaded glass, or placed within the positive column of the lamp. Sincethe antioxidant is enclosed in a glass capsule it could be present inthe inside or positive column of the lamp without affecting lampfunction.

The antioxidant material can also be incorporated in the basing cementof the lamp that holds the aluminum cap to the leaded glass portion ofthe end of the lamp. The basing cement generally comprises about 80weight % marble flour (limestone-CaO), and the balance shellac aphenolic resin binder, a solvent for blending, and a dye used to colorthe cement. The cement is dispensed through a feeder into the base andheated to cure once assembled with the lamp. The curing drives off thesolvent and solidifies the cement. The antioxidant is blended with thecement components and incorporated into a lamp manually or by automatedmanufacturing equipment. The antioxidant material is released only whenthe lamp is destroyed or crushed in preparation for TCLP testing. Inthis method the active antioxidant material is always exterior to thepositive column of the lamp.

Another method for incorporating the active antioxidant material in thelamp structure is to admix it with an inert water soluble adhesivecarrier or binder. Gums and gelatins have been used as such adhesivesand binders. The nature of the gums and gelatins is that they adhere tosurfaces when heated. The composition containing the antioxidantmaterial can be placed on the inner surface of the aluminum end cap as aring or discrete button. When the lamp is crushed and exposed to anaqueous environment or placed in the TCLP solution, the water solublebinder allows the antioxidant to be released quickly.

What is claimed is:
 1. A mercury vapor discharge lamp comprising anenvelope of light transmitting glass which contains, an inert gas and anamount of elemental mercury, a pair of electrodes for establishing anarc discharge, and an effective amount of an antioxidant.
 2. The mercuryvapor discharge lamp of claim 1 wherein the antioxidant is selected fromthe group consisting of ascorbic acid and sodium ascorbate.
 3. The lampaccording to claim 1 in which the antioxidant is ascorbic acid, sodiumascorbate, calcium ascorbate, ferrous sulfate, ferrous oxide, ferroustatrate, ferrous citrate, ferrous gluconate, or ferrous chloride and ispresent in an amount of about 0.05 to about 10 grams per lamp.
 4. Thelamp according to claim 1 further comprising at least one base end capwhich defines a cavity having an inner surface, and which is secured tothe lamp envelope by a basing cement, the antioxidant being disposedwithin said cavity.
 5. The lamp according to claim 4 in which theantioxidant is admixed with the basing cement.
 6. The mercury vapordischarge lamp according to claim 5 wherein the antioxidant is carriedon the inner surface of the cavity by means of an inert water solublebinder.
 7. A method for preventing the formation of leachable mercurycompounds in mercury vapor discharge lamps which comprises incorporationinto the lamp structure of an effective amount of an antioxidant.